An electrolyte circulation type battery such as a redox flow battery is one of large-capacity storage batteries. In a redox flow battery, a positive electrolyte and a negative electrolyte are supplied to a cell having a membrane and a positive electrode and a negative electrode facing each other with the membrane interposed therebetween, to charge and discharge the battery. For the electrolytes, an aqueous solution containing a metal ion whose valence varies by oxidation-reduction is commonly used. As the redox flow batteries, for example, an iron-chromium-based redox flow battery using an iron ion aqueous solution for the positive electrolyte and a chromium ion aqueous solution for the negative electrolyte, and a vanadium-based redox flow battery using a vanadium ion aqueous solution for the positive and negative electrolytes are well-known (see e.g., Patent Documents 1 to 3).
FIG. 8 is a schematic diagram for illustrating an electrolyte circulation type battery (redox flow battery). A redox flow battery 100 includes a cell 110. Cell 110 is partitioned into a positive electrode cell 112 and a negative electrode cell 113 by a membrane 111 through which ions can pass. Positive electrode cell 112 contains a positive electrode 114, and negative electrode cell 113 contains a negative electrode 115. Redox flow battery 100 further includes, for each of the positive electrode and the negative electrode, an electrolyte tank 120 for storing the electrolyte, a circulation path 130 for circulating the electrolyte between electrolyte tank 120 and battery cell 110 (positive electrode cell 112, negative electrode cell 113), and a circulation pump 140 for circulating the electrolyte through circulation path 130. Circulation path 130 includes a go conduit pipe 131 for feeding the electrolyte from electrolyte tank 120 to battery cell 110 (positive electrode cell 112, negative electrode cell 113), and a return conduit pipe 132 for feeding the electrolyte from battery cell 110 (positive electrode cell 112, negative electrode cell 113) back to electrolyte tank 120. In redox flow battery 100 shown in FIG. 8, a vanadium ion aqueous solution is used by way of example for the positive and negative electrolytes. In FIG. 8, solid line arrows in the battery cell indicate charge reaction, and broken line arrows indicate discharge reaction.
A cell of a redox flow battery is commonly utilized in a form referred to as a cell stack. The cell stack includes a plurality of stacked cells, each cell including a membrane, and a positive electrode and a negative electrode facing each other with the membrane interposed therebetween. FIG. 9 is a schematic diagram for illustrating the cell stack. A cell stack 200 employs a cell frame 210 including a bipolar plate 211 and a frame 212 for fixing bipolar plate 211. Cell stack 200 includes a plurality of cells stacked with cell frames 210 interposed therebetween, each cell including positive electrode 114, membrane 111 and negative electrode 115 stacked on one another. That is, one cell is formed between cell frames 210 (bipolar plates 211), and in the space between cell frames 210 (frames 212), a negative electrode (negative electrode cell) and a positive electrode (positive electrode cell) of adjacent cells are arranged on front and rear sides with bipolar plate 211 interposed therebetween. In order to supply and discharge the electrolyte to and from each electrode, frame 212 of cell frame 210 is provided with liquid supply manifolds 213, 214 and liquid discharge manifolds 215, 216 through the front and rear surfaces, and guide grooves formed alternately between the front and rear surfaces to guide the electrolyte from each manifold to each electrode. In some cases, a protection plate made of plastic (not shown) is arranged to cover the guide grooves to prevent direct contact between the guide grooves and membrane 111, thus reducing the likelihood of breakage of membrane 111 after the stacking Then, a pair of end plates 220 is arranged on opposite sides of the stacked body including the plurality of cells stacked with cell frames 210 interposed therebetween, each cell including positive electrode 114, membrane 111 and negative electrode 115, and both end plates 220 are clamped in a stacking direction of the stacked body by a clamping mechanism 230 such as bolts, to form cell stack 200 (e.g., paragraphs 0004 to 0005, FIG. 9 in Patent Document 1).
For the cell frame described above, a bipolar plate made of plastic carbon (e.g., graphite-containing resin) and a frame made of plastic (e.g., vinyl chloride) are often used. This cell frame is usually assembled by holding a peripheral portion of the bipolar plate between a pair of frames, and integrating the frames and the bipolar plate together by adhesion with an organic solvent (e.g., paragraph 0028 in Patent Document 3). In this case, the pair of frames constitutes the frame. By the adhesion between the frames and the bipolar plate with an organic solvent, a sealing structure is formed that seals a space isolated by the bipolar plate between the frames.